Currently, the prevailing 3D projection optical systems commercially used are active stereoscopic displaying systems. In an active stereoscopic displaying system, a projector having a refreshing frequency of about 120 Hz is used to output a left-eye frame and a right-eye frame alternately and, meanwhile, a pair of LCD stereoscopic eyeglasses switched on alternately is used for synchronous switching so that separation of the left-eye frame and the right-eye frame from each other is achieved. This delivers a good stereoscopic effect. However, the active stereoscopic eyeglasses have a relatively high cost, frequent switching of the stereoscopic eyeglasses makes the eyes uncomfortable, and propagation of light through the LCD stereoscopic eyeglasses leads to significant loss in brightness. Furthermore, in order to achieve the 3D function, the refreshing frequency of the microelectromechanical system (MEMS) mirror chip must be at least 120 Hz, which represents a high requirement on the refreshing frequency of the chip.